Volumetric grain boundary, or intergranular attack (IGA) is a type of stress corrosion in metals that may result in cracking under a combination of high tensile stresses and corrosive environments.
In stainless steel, sensitization (weakening of bonds) results from heating nonstabilized stainless steel, causing precipitation of chromium-rich carbides along grain boundaries. These grain boundaries are thus susceptible to corrosion under certain operating conditions. In this type of corrosion, IGA initiates at a free surface and continues propagating as a plane front into the material so that a layer of IGA, whose severity is more or less uniform over a plane but which varies with depth, is formed. The cohesion of grain boundaries in the region of IGA in the vicinity of the surface may be severely weakened to the extent that little cohesive strength remains. For a surface in this state, thermal stresses associated with welding can be sufficiently large to pull grains apart, causing crack initiation sites to form. For stainless steel, tensile stresses that may cause cracking include residual stresses due to welding or fabrication, and other thermally-induced stresses. Environments that cause stress-corrosion cracking include aqueous solutions of chlorides, flourides, hydroxide ions, or dissolved oxygen. Stress-corrosion cracking may also occur in high-temperature water or boiling water nuclear power plants.
Although various methods exist for controlling sensitization, it is frequently difficult to control or predict. Sensitized steel, during use, can be exposed to conditions that promote IGA. Knowledge of the presence and extent of IGA, therefore, would be beneficial to alleviate potential problems.
Several systems for detecting flaws in metals using acoustic waves and/or eddy currents are known in the art. For example, U.S. Pat. No. 4,184,373 to Evans generally discloses a system for ultrasonically evaluating adhesive bonds between metal surfaces by comparing various echo amplitudes. Rettig U.S. Pat. No. 3,946,600 and Votava U.S. Pat. No. 4,428,236 disclose measurement of acoustic pulses emitted by the corrosion process in metals to predict failure and to locate a corroded area. Tait U.S. Pat. No. 3,939,404 and Howard U.S. Pat. No. 4,450,405 are representative of patents that generally show the use of eddy current testing to detect defects in metals.